In the early days of ultrasound imaging, an ultrasound transducer was mounted on an articulated arm that forced the transducer and the transducer beam to move within a plane. The arm included sensors for measuring the angular position of each of its joints. This angular position data was processed to determine the position of the transducer and the beam direction each time that a one-dimensional echo acquisition was performed. From such measurements, a two-dimensional image of the subject within the plane was assembled. Imaging systems of this type were described as static B-mode scanners.
In the mid-to-late 1970s, static B-mode scanners were gradually replaced by real time ultrasound imaging systems. In a real time system, a scanhead containing the transducer(s) is held essentially stationary, and the ultrasound beam is mechanically or electronically scanned over a series of scan lines in a scanning plane. The scanning is rapid enough so that complete scans through the scanning plane can be made at a rate of 20-30 scans per second, so that a real time, two dimensional moving image can be produced on a display monitor. Real time scanners not only produce moving images, but also avoid the awkwardness of the articulated arm required for static B-mode scanners. In addition, the cost of the mechanical arm and its associated sensors was substantial, approximately the cost of a real time mechanical scanhead, so that static scanners had little price advantage over real time mechanical scanners.